Create WooCommerce Order for new accounts - https://www.damiencarbery.com/2018/01/create-woocommerce-order-for-new-customers/

create-wc-order-for-new-accounts.php

<?php
/*
Plugin Name: Create WooCommerce Order for new accounts
Plugin URI: http://www.damiencarbery.com/2018/01/create-woocommerce-order-for-new-customers/
Description: Create an order with a downloadable product when a new account is created.
Author: Damien Carbery
Author URI: http://www.damiencarbery.com
Version: 0.1
*/

class FreeOrderForNewAccounts {
	private $debug_mode;
	private $user_id;
	private $free_product_id;

    public function __construct() {
        // Allow debug mode to be enabled when WordPress debug mode is enabled.
        if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
            $this->debug_mode = true;
        }
        // Or set it manually.
        //$this->debug_mode = false;
		
		$this->user_id = null;
		$this->free_product_id = null;
    }


    public function init() {
		add_action( 'woocommerce_checkout_update_user_meta', array( $this, 'post_checkout_create_new_customer_order' ), 10, 2 );
    }

	
	// Runs after a new customer account is created (and meta info, like billing address, saved).
	public function post_checkout_create_new_customer_order( $customer_id, $data ) {
		$this->user_id = $customer_id;
		
		// Users of this plugin must use this filter to specify the ID of the free product.
		// Usage: add_filter( 'free_order_for_new_accounts_set_product_id',function( $id ){ return 12; });
		$this->free_product_id = apply_filters( 'free_order_for_new_accounts_set_product_id', $this->free_product_id );
		if ( !is_int( $this->free_product_id ) ) {
			if ( $this->debug_mode ) {
				error_log( 'woocommerce_checkout_update_user_meta: Product ID not set.' );
			}
			return;
		}

		if ( $this->debug_mode ) {
			error_log( 'woocommerce_checkout_update_user_meta: Customer ID: ' . $this->user_id );
			error_log( 'woocommerce_checkout_update_user_meta: Data: ' . var_export( $data, true ) );
		}
		
		$customer_meta = get_user_meta( $this->user_id );
		
		if ( $this->debug_mode ) {
			error_log( 'woocommerce_checkout_update_user_meta: Customer Meta: ' . var_export( $customer_meta, true ) );
		}
		
		$this->create_order_for_free_downloads();
	}


	public function create_order_for_free_downloads() {
		$user = get_user_by( 'id', $this->user_id );
	  
		if ( empty( $user ) ) {
		  return '<p>ERROR: Could not retrieve user information.</p>';
		}
		$user_meta = get_user_meta( $this->user_id );

		$address_fields = array( 'first_name', 'last_name', 'email', 'phone', 'address_1', 'address_2', 'city', 'state', 'postcode', 'country' );
		$address = array();
		
		// Verify that billing address fields exist. Set to empty as default.
		foreach ( $address_fields as $field ) {
			$address[ $field ] =  isset( $user_meta[ 'billing_'. $field ][ 0 ] ) ? $user_meta[ 'billing_'. $field ][ 0 ] : '';
		}
		if ( $this->debug_mode ) {
			error_log( 'Order address fields: '. var_export( $address, true ) );
		}

		// Now we create the order
		$order = wc_create_order();
		if ( $this->debug_mode ) {
			error_log( 'Order created for customer ID: '. $this->user_id . ' with name: ' . $address['first_name'] );
		}

		$order->add_product( wc_get_product( $this->free_product_id ), 1);
		$order->set_address( $address, 'billing' );
		$order->calculate_totals();
		$order->update_status( 'wc-completed', 'Free Download', TRUE );

		// Setting the status to completed creates an entry in '{prefix}woocommerce_downloadable_product_permissions' table.
		// We need to change the user id from 0 to the required user. I don't know why it is not done by WooCommerce.
		global $wpdb;
		$wpdb->update( $wpdb->prefix . 'woocommerce_downloadable_product_permissions', array('user_id'=> $this->user_id ), array( 'order_id' => $order->get_order_number() ), array( '%d' ), array( '%d' ) );

		if ( $this->debug_mode ) {
			error_log( 'Order created: '. $order->get_order_number() );
		}
	}
}

$FreeOrderForNewAccounts = new FreeOrderForNewAccounts(); 
$FreeOrderForNewAccounts->init();

set-product-id-for-free-download.php

<?php
<?php
/*
Plugin Name: Set product ID for free download
Plugin URI: http://www.damiencarbery.com/2018/01/create-woocommerce-order-for-new-customers/
Description: Set the product ID for the order that is automatically created when a new account is created.
Author: Damien Carbery
Author URI: http://www.damiencarbery.com
Version: 0.1
*/

// Change the number 72 to the ID of the downloadable product.
add_filter( 'free_order_for_new_accounts_set_product_id',function( $id ){ return 72; });